Gas discharge lamps are a family of artificial light sources that operate on very different physics principles than incandescent lamps. While incandescent lamps generate light by heating a metal filament until it glows white hot, gas discharge lamps send an electric current through a special gas to generate light. Depending on the type of gas or mixture used, this either generates visible light directly or first generates ultra-violet light and then converts the ultraviolet light to visible light using phosphors such as fluorescent powders or coatings. Both types of gas discharge lamps have significant commercial applications. The former (generating light directly) are commonly used as park and roadway lighting, while the latter (generating light indirectly), particularly fluorescent lamps, have even broader applications due to their better control and rendering of the light colors generated.
Compared to conventional incandescent lamps, gas discharge lamps offer long life, low tube temperature and high light efficiency. For this reason, gas discharge lamps, particularly fluorescent lamps, provide a large percentage of today's lighting needs, even though they are more complicated to manufacture and require electronics to provide the correct current flow through the gas. The compact fluorescent lamp is perhaps the best known gas discharge lamp and is becoming an increasingly significant substitute of conventional incandescent lamps in both industrial and home applications.
The operation of a standard gas discharge lamp is accomplished by ionizing the mercury vapor enclosed in the lamp by applying a high voltage between two filaments (also called cathodes or electrodes) located at each end of the lamp. During ignition, the temperature of emissive coating of the filaments is increased to an optimum level for emission of electrons and to heat the mercury vapor near the filaments. Unlike incandescent light sources, gas discharge light sources will not be able to start if directly connected to a regular voltage resource. In addition, most gas discharge lighting sources, including the fluorescent lamps, exhibit negative resistance characteristics in operation, meaning that the lamp voltage is higher and the current lower when the lamp is operating at a lower power, and conversely, the lamp voltage is lower and the current higher when the lamp is operating at a higher power. The negative resistance characteristics may result in an unstable operation if the lamp is directly connected to the voltage source.
For these reasons, a fixture called ballast is used with a gas discharge lamp to perform necessary ignition and stabilization. Among the two major types of ballasts, namely electromagnetic ballasts and electronic ballasts, electronic ballasts are of particular importance. While an electromagnetic lamp ballast uses electromagnetic induction to provide the proper starting and operating electrical condition to power gas discharge lamp, an electronic lamp ballast uses solid state electronic circuitry to do the same. Because electronic ballasts usually use inverter style power supplies to rectify the input power and then chop it at a high frequency, they can change the frequency of the power from the standard mains frequency to 20,000 Hz or higher, which significantly increases the efficiency of gas discharge lamps and substantially eliminates the stroboscopic effect associated with fluorescent or high-intensity discharge lighting. In recent years, due to the significant efficiency improvements and further support by the rapid developments of semiconductor manufacturing technology and high-frequency switching technology, electronic ballasts are becoming more and more dominant in gas discharge lamps.
A ballast performs its functions by regulating the voltage and current during various stages of the lamp operation such as preheating, ignition and normal operation. Ballasts for gas discharge lamps are often classified according to the method of creating the ignition condition. These categories include instant-start, rapid-start, preheat-start and program-start ballasts. Depending on the type of gas discharge lamps and other design requirements, one type of ballasts may be found more suitable than another.
Instant-start ballasts skip the preheating stage and go directly to ignition and normal operation. For this reason, instant-start ballasts, including some quasi instant-start circuits (further described below) commonly used in energy efficient lighting products, do not require a control circuitry for preheating. In an instant-start ballast, the increase in filament temperature necessary for ignition is accomplished by allowing a high voltage applied across the lamp and the two filaments (e.g., 1000 volts peak) to establish an arc, thereby heating the filaments with the arc current. Circuits for this type of ballasts usually have the lamp connected across (in parallel with) a high-voltage source which is commonly a resonant circuit. A current path through the filaments is generally not provided. In fact, it is common for an instant-start ballast to have only two connections to the lamp, one at each end. A quasi-instant start circuit is similar in operation to the instant start circuit except that the quasi-instant start circuit allows the resonant current of the ballast to run through the lamp filaments (typically for disabling high-voltage generation by the resonant circuit during lamp removal), but like the instant-start circuit, does not have a preheating stage and therefore has no control for performing preheating.
In contrast, preheat-start and program-start ballasts heat the filaments separately to emission temperatures by allowing current to flow through the filaments themselves for a limited period of time (e.g., one second or less) before a moderately high voltage (e.g., 500 volts RMS) is applied across the lamp to ignite the lamp. Because circuits for these ballasts provide a current path through the lamp filaments for preheating, they may require a control circuit to perform a desired preheating procedure, and often have four connections to the lamp, two at each filament end of the lamp.
There are many circuit variations and implementations for these basic starting techniques in use in the industry today. Attributes such as cost, lamp life, ballast size, application and the number of connections to the lamp all affect the starting method adopted. For example, due to their typically lower costs and higher efficiencies, instant and quasi-instant start circuits are extensively used in the low-cost and energy-saving lighting market where the low cost and high efficiency drive the selection of the ignition method. In some cases, the instant-start operation can not only lower initial product costs but also produce slightly greater energy efficiency (i.e., light output per watt), since no filament heating power is delivered to the filament during normal operation of the lamp. For this reason, instant-start ballasts are most often selected for general office space lighting where large numbers of lamps are used and the lamps are not frequently switched on and off. For other low-cost energy-efficient applications, for example where low-cost is combined with the need for lowering output voltages during lamp removal or filament failure, the user may choose a quasi-instant start circuit.
However, the short lamp life is a major drawback of energy-saving light fixtures using instant or quasi-instant ballasts, particularly when used in situations where frequent lamp starting is required (as in bathrooms or areas where motion sensors are installed). This makes ballasts that perform some sort of preheating before ignition preferred in many applications. It is generally known that the preheat-start approach can result in significantly longer lamp life due to the separate heating of the filaments before ignition, which results in reduced degradation of the lamp filament's emissive coating during the ignition sequence.
On the other hand, preheat-start approach generally requires additional circuitry in order to reduce the filament heating power used during normal operation and hence may add additional cost (as compared to instant-start and quasi-instant start circuits). The ballasts with a preheating function currently available in the market are larger in size due to a significantly more complex circuit topologies and are expensive. It is therefore desirable to introduce a low-cost simplified circuit for implementing the preheating function into a ballast circuit. In addition, even for ballasts that do not perform preheating, it is often desirable to have a low-cost solution for features such as dimming function during normal operation.